Follow the steps in order for the best result.
In a mixing bowl, combine the kosher salt, black pepper, cayenne pepper, garlic powder, onion powder, and mustard powder. Divide this spice mixture evenly into two separate mixing bowls.
Add the all-purpose flour to one of the bowls containing the spice mixture. Mix thoroughly to combine, then set aside.
Take the chicken drumsticks and rub them generously with the reserved spice mix (from the bowl without flour). Using a fork, poke each drumstick several times to allow the flavors to penetrate the meat. Set aside.
Pour the buttermilk over the spiced chicken drumsticks, ensuring they are well coated. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, or for even better flavor, marinate overnight.
Place the flour mixture into a large, resealable plastic bag. Remove the chicken pieces from the buttermilk, and working in batches, place them into the bag. Shake the bag vigorously to ensure the chicken is completely coated in the flour mixture.
In a large, heavy-bottomed saucepan, pour enough canola oil to fill the pan about one-third of the way. Heat the oil over medium heat until it reaches 360 degrees F, using a deep-frying thermometer. If you don't have a thermometer, test the oil by dropping in a small cube of bread; it should brown in approximately 3 minutes when the oil is ready.
Carefully drop the coated chicken drumsticks into the hot oil. As they brown, turn the pieces to ensure even cooking, and avoid overcrowding the pan. Work in batches if necessary. To check the underside of a piece, lift it gently with tongs; it should be a deep golden brown.
Fry the chicken for about 15 minutes, turning occasionally, until it is crispy and brown all over. To test for doneness, cut into the thickest part of a drumstick. The juices should run clear, and the meat should be opaque throughout. If needed, transfer the chicken to a preheated 325 degrees F oven to finish cooking until fully cooked.
Transfer the fried chicken to a baking sheet lined with paper towels to drain any excess oil.
Move the crispy drumsticks to a serving platter or a bucket. Serve the fried chicken hot, at room temperature, or cold, according to your preference.